Bioware could have easily just side-lined the Inquisitor by having Solas Petrify them after removing the Anchor (perhaps he thinks this may save the Inquisitor from removal of the Veil, or wants them to suffer after they are thawed out too late to stop him, who knows). This would not be perceived as a hard-core death; it would distract the Inquisitor's remaining friends and allies with trying to find a way to free them from their stone prison, while still giving the important "world-ending" set-up to the Players; and Solas' "e-vile" plans would remain a secret until such a time where the Inquisitor was freed. Then a New DA:4 PC who has had plenty of time to be developed independently from the Solas issue, would be the one to save the Inquisitor at some point during their game. BUT the Inquisitor would be "un-thawed" in the same state they were in just before becoming a statue, where they are simply in no physical condition to stop Solas themselves after just having their arm gutted by magic. They could then logically be quietly shuffled to the side, pushed in a advisory role, or serve in a cinematic capacity only from that point on (even if the player gets to choose a bit of their dialogue for all three options). The only major difference is that the Inquisitor would have to decide on what happens to the Inquisition after they return a game later, to find out how the orginization fared in their absence.
There Bioware, that's how you go about doing such a thing with the set-up you had, if what you had in mind was the Inquisitor's "forced retirement". 
Another idea Bioware could have easily used if they wanted to introduce a new protagonist while leaving the Inquisitor alone would have been to simply have the DLC play out in exactly the same way until you meet Solas, then have him explain who he is and talk about history, save you by removing the mark... but not tell you he plans of destroying the world, only that he will do something, but vague enough that the Inquisitor doesn't just assume their former colleague/friend/lover plans on destroying the world as they know it. And he goes through the eluvian.
If they really wanted, they could have another scene (without the Inquisitor present) like the one after the base-game credits that explains he does plan on destroying the world if they wanted a sequel hook. This way, the Inquisitor might be surprised and amazed by all they learned, but doesn't assume there is any imminent danger, and can retire. A new PC can discover Solas intentions.
I would still much rather play as the Inquisitor regardless, for the opportunity of having the hero and villain have such a unique connection, but at least that would have made more sense that having them vow to stop him, start actively acting against him, and be potentially sidelined because they've been maimed. Or Maimed so they can be sidelined.
There is no retirement. I feel for those people that want to roleplay that, I truly do, but that is contrary to what the game portrays the Inquisitor as doing. Of course, this doesn't state what their involvement will be, or how, but they ARE involved in some manner.
I think a lot of the confusion comes from the epilogue slides, which show the Inquisitor doing various retired-y things, like pulling pranks with Sera or hanging out with Cullen and his/your new dog. This is admittedly odd, but after some discussion my girlfriend and I came to the conclusion that these particular slides must be set after DA4. At least, that's the only thing that made any sense, because they blatantly contradict the scene in the basement scene/vowing to stop Solas, and are also out-of-character not just for the Inquisitor, but for the Inner Circle, too.